For single-color or multicolor printing of a printing substrate—for example a single page or a belt-shaped recording material made of the most varied materials, for example paper or thin plastic films or metal films—it is known to generate image-dependent charge images on an intermediate image carrier (for example a photoconductor), the charge images corresponding to the images to be printed, and the images being comprised of regions that are to be inked and regions that are not to be inked. The regions of the charge images that are to be inked are revealed with toner as toner images on the intermediate image carrier with a developer station. The toner images thereby generated are transfer-printed onto the printing substrate in a transfer printing zone and are fixed there.
A developer fluid having at least charged toner and carrier fluid can thereby be used to ink the charge images. Possible carrier fluids are hydrocarbons, among others.
A method for such an electrophoretic printing in digital printing systems is known from WO 2005/013013 A2 (US 2006/0150836 A1, DE 10 2005 055 156 B3), for example. After the charge images of the images to be printed have been generated on the intermediate image carrier, these are inked with toner into toner images via a developer station. For this purpose a developer fluid consisting of a carrier fluid containing silicone oil and colored particles (toner) dispersed therein is used. The feed of the developer fluid to the intermediate image carrier can take place via a developer roller to which the developer fluid is supplied via a raster roller at which a chamber blade is arranged. The toner images are subsequently accepted by a transfer unit from the intermediate image carrier and are transferred onto the printing substrate in a transfer printing zone.
In this printing method, using developer fluid the process of electrophoresis is thus used to transfer toner in the carrier fluid to the printing substrate via transport units. The solid, electrically charged toner thereby travels via the carrier fluid as a transport means from one transport unit to the next, wherein the transport can be controlled by an electrical field between the transport units. The film of carrier fluid divides after the contact region (nip) between the respective transport units in the depleted region, such that the toner is deposited with high efficiency on the target transport unit. In addition to the toner charge and the electrical field, the provision of a sufficiently thick carrier fluid layer through which toner can travel is a requirement for this. Rotating rollers can be used as transport units, for example.
The separation characteristics of the film of developer fluid and the electrophoretic migration velocity of the toner in the carrier fluid depend on the physical properties of the carrier fluid, in particular on its viscosity. The optimal viscosity can thereby differ for different transfer steps. The transfer of the developer fluid from the developer station to the transfer printing zone in the electrophoresis process requires a carrier fluid with low viscosity since the toner should respectively travel to a target transport unit. The relationships in the transfer printing of the toner image onto the printing substrate in the transfer printing zone are different. Problems in particular occur here. The transfer of the toner image to the printing substrate is affected by pores and roughness in the printing substrate surface (for example paper). The pores in the printing substrate draw carrier fluid from its surface into said printing substrate, with the consequence that the adhesion of the toner images to the printing substrate is reduced. The roughness in the surface of the printing substrate provides for an air gap between the transport unit (transfer roller) from which the toner image should be transferred and the printing substrate. The adhesion of the toner image to the printing substrate is also thereby reduced and the transfer printing is hindered.
One method to reduce this problem is the transfusion method according to U.S. Pat. No. 5,555,185. Carrier fluid is thereby no longer used as a transport means in the transfer printing of the toner images to the printing substrate. Instead of this, the toner is softened by high temperatures in the transfer unit and then is adapted with pressure to the printing substrate. A large common surface between printing substrate and toner image with strong adhesion forces thereby arises. Due to these surface forces, the toner is transferred from the transport unit onto the printing substrate in the transfer unit, although the carrier fluid is not used as a transport means.
A developing process in an electrophotographic printing apparatus is known from U.S. Pat. No. 5,998,081 A. Developer that is present in solid form is filled into a developer station. The developer and the carrier in the developer as well have a melting point that is above 25° C.—room temperature is in a range from 20 to 25° C. In order to liquefy the developer, this must be heated. This must take place before the development of the charge images. After the development of the charge images on the charge image carrier into toner images, the transport of the toner images no longer takes place in carrier fluid. The transfer printing of the toner images onto a printing substrate is executed not in carrier fluid but rather using pressure.
U.S. Pat. No. 5,352,557 A describes the design of developer fluid whose carrier fluid is selected such that environmental contamination and fire risk are minimized, in particular in the fixing.